The present invention is directed generally to photo detector devices fabricated on semiconductor substrates. More particularly, the invention relates to photo detectors manufactured as part of semiconductor integrated circuits. Still more particularly, the invention concerns photo detector structures formed using BiCMOS process integration and device optimization techniques for integrated circuit designs. From an operational standpoint, the invention is directed, but not necessarily limited, to photo detector devices for infrared (IR) wireless and optical fiber applications.
The drive towards monolithic optical transceiver systems and optical fiber drivers and receivers creates a need for photo detectors that are integrated in full compatibility with main-stream silicon fabrication processes, such as state-of-the-art BiCMOS production. If such photo detectors are used for IR-wireless applications, their designs have to focus on a maximum signal-to-noise ratio. In contrast, designs for optical fiber system applications require high-frequency photo detectors.
Photodiodes operated under reverse bias conditions are one type of semiconductor optical device that have been used extensively in the past. Such devices generally have good response speed, but minimal quantum efficiency. Thickening the depletion region increases quantum efficiency but at the expense of response speed. The location of the depletion region relative to the light impinging surface also affects performance, because short wavelength light is absorbed near the device surface, while longer wavelength light reaches further below the device surface. A depletion region that is too deep will thus reduce high end spectral sensitivity, as well as overall output response.
Another semiconductor photo detector device is the phototransistor. Phototransistors are typically configured as vertical bipolar structures, but with a large asymmetry of emitter area and collector area, i.e., the collector area is much larger than the emitter area. When operated with a floating base contact, phototransistors have an advantage over photodiodes in that the photo current generated at the reverse-biased collector-base junction is increased by the current gain of the transistor. The current gain, and thus the increase of the photo response over that of a photodiode, is high. This benefit, however, comes at the expense of an increased noise equivalent power and a reduced high frequency response due to the phototransistor's inherently larger collector area. Moreover, the collector-base junction is generally located at a considerable distance from the light impinging surface so that a fraction of the electron-hole pairs generated near the surface may not contribute to the photo current, thus reducing spectral sensitivity. Non-photo-type lateral transistors are known in the art, but they generally have higher base resistance, and hence, slower response speed than vertical transistors. This stems, at least in part, from the fact that base width in lateral transistors is controlled relatively imprecisely by lithograpaphic techniques, whereas in vertical transistors, base width is controlled with comparative accuracy using deposition processes such as implantation, diffusion and the like.
The prior art thus lacks important features that are necessary to a fully functioning photo detector. What is needed is a photo detector that provides broad spectral bandwidth sensitivity and improved output signal intensity without sacrificing response speed. What is also required is a method for producing such photo detectors using existing integrated circuit fabrication processes, and especially state-of-the-art BiCMOS fabrication, in order to facilitate the incorporation of photo detector devices in integrated systems designed for applications such as optical fiber communications, IR-wireless transmission, and the like.